Technical Field
The present invention relates to a satellite radio wave receiving device, an electronic timepiece, a positioning control method, and a storage medium.
Background Art
There is a technology for receiving radio waves from positioning satellites belonging to global satellite positioning systems such as the GPS (Global Positioning System), and acquiring a radio wave receiving position (current position) and date and time. In a satellite radio wave receiving device that receives radio waves from positioning satellites, the current position and an accurate date and time can be calculated based on the receiving timing of signals (navigation messages) included in transmitted radio waves from a plurality (ordinarily four or more) of positioning satellites and information pertaining to the positions of the plurality of positioning satellites.
A navigation message is transmitted by being spectrum-spread using a pseudo-noise code (PRN code) specific to each positioning satellite, in other words, by phase-modulating (BPSK) a carrier wave by means of a PRN code that has been phase-inverted according to bit values of the navigation message. In a case where a navigation message is to be demodulated and decrypted, the PRN code type and phase (head timing of a repeatedly used PRN code) are identified, and the navigation message is inverse spectrum-spread by means of the identified PRN code.
In a case where the environment in which radio waves from positioning satellites are received is poor and a sufficient strength and CN ratio cannot be obtained, there are cases where it is difficult to capture and demodulate signals and to acquire information. There is a technology for improving the receiving sensitivity (CN ratio) in such cases by receiving and superimposing an iterated code a plurality of times. One period of a PRN code, which is available to civilians and is commonly used, is shorter than the duration of each bit value. As a technology for correcting a date and time possessed and counted even in a case where the received radio waves are weak, Japanese Patent Application Laid-Open Publication No. 2010-96672 discloses that the receiving sensitivity is improved by synchronously adding received PRN codes in an appropriate manner while giving consideration to the phase inversion of the PRN codes according to each bit value of a navigation message, and that the phase (head timing) of the PRN codes and the phase (head timing) of each bit value are identified.
Meanwhile, Japanese Patent Application Laid-Open Publication No. 2010-96672 describes a known assisted positioning technology (assisted GPS or the like) for externally acquiring temporary information of positioning satellites using a separate communication unit, rather than acquiring from navigation messages, when acquiring a current position. The technology is disclosed with which the current position is promptly identified as soon as the transmission time of radio waves from a positioning satellite is acquired by retaining the position information of positioning satellites in advance in this way.
However, although accurate date and time information is essential for reliably capturing and tracking transmitted radio waves from positioning satellites and acquiring accurate position information from the beginning, there is a problem in that the acquired date and time gradually deviate as time elapses from the previous positioning. As a result, the position can no longer be obtained quickly and appropriately from the beginning, and even if obtained there is likely to be a decline in position precision.